Klallamornis is an extinct genus of Plotopteridae, a family of large, flightless birds related to modern cormorants, darters, gannets, and boobies.

[3] In 2015, Goedert, Gerald Mayr and Olaf Vogel speculated that the remains associated with Tonsala buchanani may represent several species, on account of the lapse of 5 million years between the Whiskey Creek plotopterid and the holotype, and that some morphological characters indicated that T. buchanani differed at the genus level from T. hildegardae, the type species of Tonsala.

[5] In 2016, Goedert and Mayr described a new genus and two new species of plotopterids : Klallamornis abyssa, the type species of the new genus, based on a partial skeleton collected in 1991 by Goedert, SMF Av 610, and dated from the Late Oligocene of the Pysht Formation, with two other specimens, an isolated and fragmentary tibiotarsus and a coracoid, respectively collected in 1988 by Goedert and in 1985 by M. J. Berglund, both from the Jansen Creek Member of the Makah Formation, referred to the genus; and the tentatively assigned ?Klallamornis clarki, much larger, but only known from a tarsometatarsus collected in 1988 in the Jansen Creek Member of the Makah Formation by R. L. Clark.

The coracoid had a processus lateralis forming an omally directed hook, and the angle between it and the sternal articular facet was steeper than that of Copepteryx; the extremitas sternalis was devoid of the narrow spur present on the latter genus.

The first, taking into account the absence of the distal vascular foramen in the tibiotarsus of both Klallamornis and Olympidytes, consider them to be sister taxa; while this theory is not currently retained, other characters, like a similarly shaped femur, seems to unite the wo genus.

[2] The second, retained in a latter cladistic analysis,[14] considers that Klallamornis, due to its size, is more closely related to the large Japanese taxa Hokkaidornis and Copepteryx,[2] potentially clading with them, as:[14] Stemec Plotopterum Phocavis Tonsala Olympidytes Klallamornis Copepteryx Hokkaidornis Within the genus, and although K. abyssa and K. buchanani may be synonymous, it has been suggested that ?K.

clarki, the larger subspecies, is closely related to the Japanese giant taxa, assuming that gigantism evolved only once in tonsaline plotopterids.

clarki to the genus is only tentative[2] and not supported by precise characters, due to the paucity and the bad preservation of the remains attributed to K. buchanani and K.

The femur was shorter than that of K. abyssa, although this size difference can be caused by sexual dimorphism; satisfying distinction between the two species is difficult to establish, and they are mainly differentiated by characteristics of their femora, notably by the presence of a straighter shaft and a wide medial rim on the caudal portion of the condylus lateralis.

[6] The species name, buchanani, honours William "Bill" Buchanan, a late resident of Clallam Bay, who collected and donated several specimens to institutions.

Its inclusion within the genus Klallamornis is only speculative, due to the paucity of its remains and the absence of overlap with specimens of K. abyssa, aside from the vertebrae of the paratype.

It is known from five specimens; its holotype, LACM 129405, a left tarsometatarsus, the tentatively referred paratype SMF Av 612, a fragmentary pelvis and two associated vertebrae, the former of which shared several similarities with that of Hokkaidornis,[2] and a fragmentary humerus,[6] all collected in Late Eocene to Early Oligocene rocks from the Jansen Creek Member of the Makah Formation,[2] as well than two specimens collected in Late Eocene to Early Oligocene rocks of the Lincoln Creek Formation, a fragmentary couple of humerus associated with a thoracic vertebra, and an isolated thoracic vertebra.

Discovered in a Late Eocene strata of the Makah Formation near the Whiskey Creek locality, and potentially older than the primitive Phocavis, it was only referred to ?K.

[6] The rapid evolution and diversification of plotopterids can be explained by the occurrence of large-scale climatic and environmental changes in the North Pacific between the end of the Eocene and the early Oligocene.

[4][2] From the deposits of the Pysht Formation where Klallamornis abyssa and K. buchanani were collected, several early whales have been discovered,[4] like the stem-mysticetes Borealodon osedax[16] and Sitsqwaik cornishorum,[17] and the aetiocetid Fucaia goedertorum.

[2] The reason for the use of gastroliths by plotopterids, as well as modern penguins, is still unknown,[4] although some have proposed that modern penguins swallow pebbles to aid in their digestion by crushing and grinding their food, to ballast themselves in the water column and regulate their buoyancy, to eliminate their stomach parasites, or to avoid the atrophy of their stomach during the long fasting periods caused by the molting process and the reproduction.

Global changes in the temperature of the oceans and the erosion of the offshore volcanic islands on which they nested may have caused the extinction of large-sized, and eventually all genera of plotopterids.

[6] It has also been suggested that the preying and competition for food and shelter exerted by the diversification of early seals, like Enaliarctos or Pinnarctidion, into the North Pacific may have been detrimental for the survival of plotopterids.

[4][2] In Late Miocene deposits from California, they were replaced by another unrelated clade of flightless marine birds, the Mancallinae or Lucas auks.